U.S. patent application number 10/257302 was filed with the patent office on 2003-11-06 for variable speed hydraulic pump.
Invention is credited to Barani, Moe K., Bishop, Michael B., Flanary, Ron, Knuth, Bruce E., Mentik, Laurentius A.G., Piedl, Martin, Pili, Roger, Steber, George R..
Application Number | 20030206805 10/257302 |
Document ID | / |
Family ID | 26893172 |
Filed Date | 2003-11-06 |
United States Patent
Application |
20030206805 |
Kind Code |
A1 |
Bishop, Michael B. ; et
al. |
November 6, 2003 |
Variable speed hydraulic pump
Abstract
The invention provides a variable speed hydraulic pump designed
to operate at a maximum horsepower throughout its pressure range by
adjusting motor speed according to motor load parameters. In
particular, the variable speed hydraulic pump includes a hydraulic
pump unit coupled to a variable speed electric motor by a drive
unit and to a hydraulic fluid tank for pressurizing and pumping
hydraulic fluid when operated by the motor. A motor controller is
electrically connected to the motor to supply drive signals to the
motor based on electrical characteristics of the drive signals
which are dependent on the load exerted on the motor. Suction from
the load is provided by both the main pump and a bidirectional
supercharging pump by reversing the direction of the motor and
shifting a 4/3 valve to connect the main pump inlet to the load and
its outlet to tank. In addition, the controller reduces the motor
speed at the maximum rated pressure to just maintain the pressure,
to reduce the amount of fluid pumped through the maximum pressure
relief valve.
Inventors: |
Bishop, Michael B.;
(Madison, WI) ; Pili, Roger; (Madison, WI)
; Knuth, Bruce E.; (Marshall, WI) ; Barani, Moe
K.; (Radford, VA) ; Flanary, Ron; (Blacksburg,
VA) ; Mentik, Laurentius A.G.; (Haaksbergen, NL)
; Steber, George R.; (Mequon, WI) ; Piedl,
Martin; (Blacksburg, VA) |
Correspondence
Address: |
QUARLES & BRADY LLP
411 E. WISCONSIN AVENUE
SUITE 2040
MILWAUKEE
WI
53202-4497
US
|
Family ID: |
26893172 |
Appl. No.: |
10/257302 |
Filed: |
March 12, 2003 |
PCT Filed: |
April 13, 2001 |
PCT NO: |
PCT/US01/12221 |
Current U.S.
Class: |
417/44.2 ;
417/199.1; 417/326 |
Current CPC
Class: |
F04B 2201/1203 20130101;
F04B 2203/0209 20130101; F04B 2203/0208 20130101; F04B 2203/0201
20130101; F04B 1/0426 20130101; F04B 2203/0207 20130101; F04B 49/20
20130101; F04B 23/103 20130101 |
Class at
Publication: |
417/44.2 ;
417/326; 417/199.1 |
International
Class: |
F04B 049/06 |
Foreign Application Data
Date |
Code |
Application Number |
May 11, 2000 |
US |
09568763 |
Claims
We claim:
1. A variable speed hydraulic pump, comprising: a variable speed
electric motor; a hydraulic pump unit coupled to the electric motor
and a hydraulic fluid source for pumping hydraulic fluid when
operated by the motor; and a motor controller electrically
connected to the motor for supplying an electrical drive signal to
the motor in response to an electrical motor load signal
characteristic of said drive signal which varies dependent on the
torque exerted by said motor, said electrical motor load signal
characteristic being detected by said motor controller, and wherein
said motor controller supplies said electrical drive signal so as
to output a constant power to said pump over substantially the
entire operating pressure range of said pump.
2. The variable speed hydraulic pump of claim 1, wherein the motor
load signal characteristic is motor current.
3. The variable speed hydraulic pump of claim 1, wherein the motor
load signal characteristic is motor current phase angle.
4. The variable speed hydraulic pump of claim 1, wherein the motor
controller is programmed to decrease the speed of the motor when
the motor load signal characteristic corresponds to a maximum rated
pump pressure so as to maintain said maximum rated pump
pressure.
5. The variable speed hydraulic pump of claim 1, wherein the motor
is coupled to the hydraulic pump unit by a drive unit that
positively drives the hydraulic pump unit in compression and
suction.
6. The variable speed hydraulic pump of claim 5, further comprising
a bidirectional supercharging pump driven by said motor, and
wherein the motor can be reversed so that the supercharging pump
can suck fluid from a hydraulic load supplied with said fluid by
the hydraulic pump unit.
7. The variable speed hydraulic pump of claim 6, wherein the
hydraulic pump unit is a piston pump.
8. The variable speed hydraulic pump of claim 7, wherein the
supercharging pump is a gear pump.
9. The variable speed hydraulic pump of claim 7, wherein the
hydraulic pump unit includes a housing defining a plurality of
piston chambers in communication with inlet and outlet ports and
housing a plurality of pistons movable in succession as the drive
unit is rotated by the motor.
10. The variable speed hydraulic pump of claim 9, wherein the
piston chambers are lined by piston inserts.
11. The variable speed hydraulic pump of claim 9, further including
a fluid tank containing hydraulic fluid in communication with the
pump chambers through hydraulic lines.
12. The variable speed hydraulic pump of claim 9, wherein the drive
unit includes a shaft coupled at one end to the rotor of the motor,
the shaft supporting an eccentric to which is fixed a cam
element.
13. The variable speed hydraulic pump of claim 12, wherein the
drive unit includes a weight fixed to the shaft to counterbalance
the eccentric.
14. The variable speed hydraulic pump of claim 12, wherein the cam
element engages heads of the pistons in a plurality of slots spaced
about the cam element such that the cam element moves the pistons
into and out of the piston chambers.
15. The variable speed hydraulic pump of claim 14, wherein the
slots are defined by flanges extending radially outward from the
cam element.
16. The variable speed hydraulic pump of claim 15, wherein the cam
element includes an annular surface for contacting the pistons.
17. The variable speed hydraulic pump of claim 15, wherein the cam
element includes multiple flat surface for contacting the
pistons.
18. The variable speed hydraulic pump of claim 5, further
comprising a valve that in a retract mode connects an intake port
of said hydraulic pump unit with the hydraulic load normally
supplied by said hydraulic pump unit.
19. The variable speed hydraulic pump of claim 18, further
comprising a bidirectional supercharging pump connected to an inlet
port of said hydraulic pump unit and driven by said motor, and
wherein the motor can be reversed so that the supercharging pump
can suck fluid from a hydraulic load supplied with said fluid by
the hydraulic pump unit.
20. The variable speed hydraulic pump of claim 19, wherein said
supercharging pump is limited by a pressure relief valve to a
pressure output of less than 100 psi.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Serial No. 60/197,789 filed Apr. 14, 2000, and
is a CIP of U.S. patent application Ser. No. 09/568,763 filed May
11, 2000.
STATEMENT OF GOVERNMENT SPONSORED RESEARCH/DEVELOPMENT
[0002] Not applicable.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates to hydraulic pumps, and in particular
to a variable speed hydraulic pump.
[0005] 2. Discussion of the Prior Art
[0006] Hydraulic pumps are useful for providing power to a work
producing device by means of hydraulic fluid under pressure.
Hydraulic pumps are used to supply hydraulic fluid pressure for
lifting, pressing, punching, and other mechanical operations when
used with suitable hydraulic presses, punches, cylinders, and other
devices.
[0007] Pumps which provide the fluid for these applications
typically have a nonlinear flow versus pressure characteristic
curve. At low pressures, the flow is high and as the pressure
increases, at a certain pressure the flow is drastically reduced.
Having a high flow at low pressures greatly reduces cycle times for
improved productivity and produces high performance for industrial
applications, and the ability to produce high pressures, albeit at
lower flows, makes the pump suitable for high force
applications.
[0008] Pumps of this type are typically a two stage design,
utilizing a first stage gear pump and a second stage piston pump.
The low pressure pump is either a gear pump, gerotor pump, or a
large piston pump. The second stage pump is usually a relatively
small diameter piston pump capable of producing high pressures.
Below 1000 psi, the first stage pump supplies the oil at a high
flow rate. When the pressure reaches about 1000 psi and above, the
first stage bypass valve opens to relieve pressure from the first
stage pump to the tank pressure, and the second stage pump will
supply the fluid at these higher pressures.
[0009] The flow of the first stage excess output (the flow not
delivered to the load) over the bypass valve creates heat and, in
excess, breaks down the oil. Heat exchangers were often required on
such pumps to preserve the hydraulic fluid quality. When the second
stage pump reaches the maximum pressure, typically around 10,000
psi, the flow from the second stage pump is dumped over a relief
valve to limit the pressure. This dumping also creates a large
amount of heat because the heat generated is a function of the flow
and pressure. These flow characteristics are illustrated in FIG. 7
as the current (prior art) pump. In one aspect, the present
invention addresses the problem of excess heat developed in the
fluid by dumping fluid over the pressure relief valve at the
pressure limit of the pump.
[0010] A two stage design is used because such pumps are typically
driven by a constant speed electrical motor operating in an open
loop mode. An example of a pump having all of these characteristics
is the prior art Enerpac 20-Series electric pump, available from
Enerpac, a unit of Actuant Corporation, Milwaukee, Wis.
[0011] Attempts have been made to make a pump serve low pressures
and high pressures with a single pump by varying the speed of the
motor which drives the pump. Such attempts have involved measuring
the pressure output of the pump, and using that as an input to the
motor controller to set the speed of the pump. Requiring a pressure
detector adds expense to the pump, making it impractical for many
applications.
[0012] In a related application, variable displacement axial piston
pumps are also currently available. The axial pistons run on a
swashplate. The swashplate is hinged to allow the pistons to change
their displacement in the piston bores. When the swashplate is at a
large angle from 90.degree. to the pistons, the pistons have long
strokes and therefore large displacements. When the swashplate is
at a small angle from 90.degree., the pistons have short strokes
and therefore small displacements. When the swashplate is at
90.degree. to the pistons the pistons do not stroke and no flow is
produced. To make this pump pressure compensated, a piston is
attached to the swashplate that senses system pressure. This pump
will provide a near constant horsepower system. These pumps are
known in the industry and are similar to Rexroth A10VSO. These
pumps are generally limited to lower pressures because of the
frictional forces that are applied to the swashplate at high
pressures.
[0013] Oftentimes, hydraulic pumps are used to power single acting
hydraulic cylinders. Such cylinders are connected to a single
hydraulic line, which provides fluid under pressure to extend or
retract the cylinder, and the cylinder is moved in the other
direction by a spring when the pressure is relieved. If the
hydraulic line is long, or in very cold temperatures in which the
hydraulic fluid becomes viscous, the spring may not be strong
enough to return the cylinder. In such cases, one method of
returning the cylinder is to apply suction to the fluid in the
hydraulic line connected to the cylinder. It is an object of the
present invention to provide a pump adapted for this as well.
SUMMARY OF THE INVENTION
[0014] The invention provides a variable speed hydraulic pump
designed to operate at a maximum horsepower throughout it pressure
band. In particular, the variable speed hydraulic pump includes a
hydraulic pump unit coupled to a variable speed electric motor and
to a hydraulic fluid source for pressurizing and pumping hydraulic
fluid when operated by the motor. A motor controller is
electrically connected to the motor to supply drive signals to the
motor based on elecrtrical characteristics of the drive signal
which are dependent on the motor load so as to provide an
approximately constant horsepower output of the motor.
[0015] The invention therefore provides a hydraulic pump that uses
a single stage pump and a variable speed motor. A pump of the
invention provides high flow at low pressure and flow that varies
inversely proportional to pressure without using a pressure
transducer to provide an input to the motor controller. Ideally,
the motor speed is varied so as to maximize the utilized horsepower
of the pump motor at any given pressure, so that the load is served
as quickly as possible by the pump. A pump motor controller is
programmed to monitor the motor current and/or phase angle, which
is related to the driven load, i.e., the pressure output of the
pump, so as to enable the motor speed to be controlled in
accordance with pump pressure without the need for a separate pump
pressure sensor and associated electronics.
[0016] At low pressures, the motor spins at high speed to produce
high flow. Since the pressure is low, the torque load on the motor
is minimal and relatively little current is drawn by the motor. As
the pressure, and therefore the torque and current draw, increases,
the speed of the motor is gradually reduced in accordance with the
increased load, preferably being reduced so as to maintain the
power output relatively constant, at or near the maximum power
output of the pump. The pump therefore supplies high pressure at a
reduced flow, although not as reduced, particularly for
intermediate pressures, as the prior two stage pumps.
[0017] In practicing the invention, a motor controller is used that
monitors the current drawn by the motor and/or the phase angle.
These parameters are roughly proportional to the pressure output of
the pump, since higher pressures increase the torque on the pump
drive motor, which increases the current draw and increases the
phase angle. As the current draw goes up, the speed is
correspondingly reduced by the controller to maintain the power
output by the pump relatively constant.
[0018] In practicing the invention, since the pump is controlled by
an electronic controller, the prior art pump's first stage bypass
valve can be eliminated. Elimination of this bypass valve produces
additional benefits since heat generated by the valve and the
resulting-destruction of hydraulic oil is eliminated.
[0019] The invention also results in higher flow rates, at a given
maximum horsepower rating, particularly for pressures that are
above the first stage maximum pressure and below the second stage
maximum pressure. The prior art pump has a flow curve that drops
off at 1000 psi and remains constant until maximum pressure. This
means that the flow at 3000 psi is the same as the flow at 10,000
psi. The new pump maximizes the flow at each pressure. For example,
the flow at 3000 psi would be over 3 times greater than the flow at
10,000 psi.
[0020] It is preferred to use a gear pump in series to pre-charge a
piston pump which is driven to supply the load. The gear pump
provides a relatively low presssure (up to 100 psi for example) to
provide a flow to the main pump with a pressure and flow rate that
varies proportionally with pump speed so as to precharge the main
pump and inhibit or prevent cavitation. At high speed the pressure
is higher to help fill the main pump in less time. At low speeds,
the pressure is lower, but cavitation is not a problem at low
speeds.
[0021] Another preferred aspect of the invention is positive return
of the piston or pistons of the main pump. In the prior art, the
pistons were driven in reciprocation by a cam eccentric journalled
to the drive shaft of the pump, and each piston was biased against
the outer surface of the eccentric by a spring. The spring force
had to be high to maintain the pistons in contact with the cam at
high speeds, but this high force wastes power in the system. In a
preferred aspect of the invention, the pistons are coupled to the
eccentric so that the eccentric not only drives them in compression
(toward top dead center) but also positively returns them in
suction (toward bottom dead center), so the motor is not wasting
power compressing the springs. The ability of the main pump to
produce a subatmospheric pressure (suction) is also improved.
[0022] In this aspect, the pump motor is preferably reversible, and
provision is made in the pump hydraulic circuit to create a vacuum
in the outlet line by reversing the direction of the motor to drive
a bidirectional supercharging pump in reverse, to aid removal of
hydraulic fluid from the outlet line quickly, thereby resulting in
fast retraction of hydraulic cylinders or other loads supplied by
the pump. Preferably, both the main pump and the supercharging pump
contribute to the suction pressure which provides for fast
retraction.
[0023] As another preferred feature of the invention, the
electronic controller that controls the pump drive motor is
programmed to reduce the flow by reducing the speed of the pump
drive motor at the maximum pressure of the pump, e.g., at 10,000
psi, to reduce the amount of fluid which is pumped over the maximum
pressure relief valve, and thereby reduce heating of the fluid. The
flow that is produced is enough to keep the system at pressure and
make up for any leakage in the system.
[0024] These and other objects and advantages of the invention will
be apparent to those skilled in the art from the detailed
description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a schematic block diagram of a variable speed pump
incorporating the invention;
[0026] FIG. 2 is a physical schematic diagram illustrating the main
components of a pump of the invention, hydraulically connected to a
single acting hydraulic actuator;
[0027] FIG. 3 is a perspective view of the main pump drive,
illustrated along with one pumping unit;
[0028] FIG. 4 is a top plan view of FIG. 3;
[0029] FIG. 5 is a cross-sectional view from the plane of the line
5-5 of FIG. 4;
[0030] FIG. 6 is a schematic of a hydraulic circuit for practicing
the invention;
[0031] FIG. 7 is a graphical representation of pump flow versus
pressure comparing a typical prior art two stage pump to a pump of
the invention of comparable maximum capacity;
[0032] FIG. 8 is a top view of an alternate pump drive with five
pump units; and
[0033] FIG. 9 is a cross-sectional view along line 9-9 of FIG.
8;
[0034] FIG. 10 is a perspective view of a shaft mounted eccentric
and ring cam for the embodiment of FIG. 8; and
[0035] FIG. 11 is a perspective view similar to FIG. 10 of another
alternate embodiment with a five-sided cam.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] Referring to FIG. 1, there is illustrated a block diagram of
the variable speed pump. The block labeled 3 corresponds to the
variable speed pump invention. The electrical power supply 1 to the
pump is obtained through standard electrical distribution such as
120 VAC, 240 VAC, or other voltages and may be single phase or
three phase in nature. It is shown supplying the pump 3 with
electrical power by means of line 2. The output of the pump is a
hydraulic line 13 that feeds a hydraulic tool 12, for example. The
pump also has provisions for a human operator interface, i.e., a
remote control pad, as shown by block 7. Block 7 provides inputs to
the pump 3 such as power on, power off, forward, reverse and so on.
These functions are communicated to the pump by means of line
8.
[0037] The variable speed pump 3 has three main components
indicated by the motor control system 4, the electrical motor 9 and
the hydraulic pumping unit 14. The pump also has a tank 11 to
supply hydraulic fluid to the pump via line 15, and to store
hydraulic fluid returned from the load.
[0038] The motor control system 4 has inputs for electrical power
via line 2, and a human operator interface via line 8. The motor
control system 4 is electrically connected via lines 5 and 6 to the
motor 9. It can monitor the motor current to determine the load of
the electrical motor 9 via line 6. A drive signal for the motor 9
is generated in the controller 4 based on the load of the motor.
One means of doing this is by monitoring the motor current. The
motor current is a relative indicator of the shaft torque load on
the motor, which in turn is an indicator of the pressure on line 13
being delivered by the pumping unit 14. Thus the speed of the motor
9 can be varied (which varies the flow of the pump), depending on
the output pressure of the pumping unit 14. At low pressures the
controller 4 provides a signal which causes the motor 9 to run at
high speed via line 5. At higher pressures, the controller 4
provides a signal which causes the motor to run at progressively
lower speeds, inversely proportional to the pressure, so as to
produce a relatively constant power output, which is proportional
to the product of pressure times flow rate. The motor 9 is directly
connected to drive the pumping unit 14, i.e., the motor drive shaft
is connected to the pump drive shaft by a direct coupling, or a
belt or chain drive, so that as the motor speed is varied the pump
speed is also varied. In some cases, because of motor speed or
torque limitations, a reduction may be provided, for example a
gearbox, between the motor and the pump, which will, in that case,
produce a pump speed that is proportional to the motor speed.
[0039] Any type of electrical motor in which characteristics of the
current drawn by the motor vary with the pressure output of the
pump may be used to practice the invention. Such motors include AC
induction motors, switched reluctance motors, universal motors, DC
and DC brushless motors. Characteristics other than the magnitude
of the current may be monitored to give an indication of the
torque, and therefore the pressure, produced by the motor. For
example, the phase angle may be measured or calculated and used as
such an indication. Motor controllers for measuring and monitoring
current characteristics and relating them to the torque produced by
the motor, to control the torque or speed of the motor, are well
known and commercially available. For example, a dedicated constant
horsepower drive could be used to practice the invention, or a flux
vector drive, such as the "Impact" drive (for an AC induction
motor) from Rockwell Automation, Milwaukee, Wis. or a motor/drive
system (for a switch reluctance motor) available from Mavrick
Motors, Mentor, Ohio, could be programmed to provide constant
horsepower over the entire operating range.
[0040] To provide the most efficient performance of the pump, the
speed of the pump is controlled by the controller to yield the
maximum power output of the motor, and therefore of the pump, at
each operating pressure. Thus, for a given horsepower motor, for
example, 11/2 hp, the controller monitors the current
characteristics, and adjusts the speed, i.e., it adjusts the
frequency, phase angle and voltage of the electrical signal which
drives the motor, to yield 11/2 hp (disregarding the negligible
horsepower required to drive the supercharging pump with the same
motor), according to the equation EP=KST, where BP is horsepower, K
is a proportionality factor, S is speed and T is torque. Therefore,
at any pressure demanded by the hydraulic load, certain current
characteristics will be detected by the motor controller, and the
controller will deliver power to the motor to drive it at the
maximum speed it is capable of (at its horsepower rating) at that
pressure. The maximum flow rate which the motor 9/pump 14
combination is capable of producing at that pressure at the
horsepower rating of the motor 9 will therefore be delivered to the
load.
[0041] Referring to FIG. 2, the pump 3 also includes a housing 20
and a valve 22. As illustrated in FIG. 2, the tool 12 is a single
acting hydraulic cylinder. When the valve 22 and motor 9 are in the
advance mode, the pump 3 will supply pressurized fluid to the
cylinder 12. When the valve 8 is in the retract mode and the motor
9 is running backwards, the pump 3 will pump fluid from the
hydraulic hose 13 and will retract the cylinder 12. The motor
control system 4, pumping unit 14 and tank 11 are housed in the
housing 20.
[0042] FIGS. 3-5 illustrate a mechanical drive for the pumping unit
14. As is common in such drives, a shaft 30, which is driven by the
motor 9, has an eccentric 38 on which is journalled a hex cam 40 by
a bearing 42. The hex cam 40 has six sides as is common, but the
cam 40 is unique in that three of its sides have flanges 32 that
define T-shaped slots 34. The three sides of the cam 40 that have
the flanges 32 are equi-angularly spaced from one another, and
receive in each slot 34 a head 36 of a piston 44 which reciprocates
in a pumping chamber of a piston block 46. There are three blocks
46 and associated pistons 44 equally spaced around the cam 40, with
the head of each piston 44 received in a different one of the slots
34, although only one block 46 and associated piston 44 is
illustrated in FIGS. 3-5. Any number could be provided. Appropriate
check valves permit flow into the pumping chamber inside the block
2 on a suction stroke and flow out of the chamber on a pumping
stroke, as illustrated and as is well known in the art.
[0043] As the shaft 30 is rotated, eccentric 38 orbits around the
axis of the shaft 30. The hex cam 40 is not allowed to rotate but
does orbit with the eccentric 38, causing the pistons 44 (only one
shown, as explained above) to reciprocate in their corresponding
valve blocks 46. When the shaft 30 is rotating, the pistons 44 will
separate from the abutting faces of the hex cam 40 during the
retract motion. Most pumps use a spring to keep the face of each
piston 44 in contact with the face of the hex cam 40. At high
speeds a high spring force is required to keep the piston in
contact with the cam which creates inefficiencies in the pump.
Springs are not used in the preferred embodiment, since the flanges
32 pull the shoulders of the heads 36 of the pistons 44 to retract
the pistons 44 on their suction strokes.
[0044] FIGS. 8, 9 and 10 illustrate an alternate embodiment of the
mechanical drive. Like elements in this embodiment are referred to
in the drawings with similar numerals as in the above described
embodiment although with the suffix "A". In particular, a shaft
30A, driven by the motor 9, mounts a separate eccentric 38A by a
key or dowel pin 100. A ring cam 40A is journalled to the eccentric
38A by a bearing 42A held in place by a washer 101 and snap ring
102. Although circular instead of hex shaped, the cam 40A is like
that in the above embodiment in that it has a flange 32A, albeit
only at one side, that includes five axial tabs 103 that define
slots 34A which receive flanged heads 36A of pistons 44A which
reciprocate in a pumping chamber of piston blocks 46A lined by
steel piston sleeves 104 preferably scaled at the bottom by copper
gaskets 105 and having a threaded outer diameter that engages with
threaded openings in block 46A. There are five blocks 46A and
associated pistons 44A equally spaced around the cam 40A and
defined by annular housing 107. Appropriate check valves 106 and
108 respectively permit flow into the pumping chamber inside the
block on a suction stroke and flow out of the chamber on a pumping
stroke, as illustrated and as is well known in the art. A generally
semi-circular counterweight 110 is mounted to the shaft 30A by the
dowel pin 100 at the short side of the eccentric 38A to balance the
weight of the eccentric 38A and reduce vibration when the shaft 30A
is rotated.
[0045] Like the embodiment of FIGS. 3-5, as the shaft 30A is
rotated, the eccentric 38A orbits around the axis of the shaft 30A.
The cam 40A is not allowed to rotate but orbits with the eccentric
38A, causing the pistons 44A to reciprocate in their corresponding
cylinder blocks 46A. When the shaft 30A is rotating, the pistons
44A will be consecutively forced into the pump chambers during
their pump strokes by contact with an annular surface 109 of the
cam 40A as the eccentric orbits toward each piston. Again like the
first embodiment, springs are not used to retract the pistons 44A
since the flange tabs 103 pull the shoulders of the heads 36A of
the pistons 44A on their suction strokes. The piston heads 36A
include a liner 111 preferably made of a hard plastic, such as a
polyamide-imide (commercially available as Torlon.RTM. a registered
trademark of Amoco Performance Products), for reducing friction and
noise when the piston heads 36A are engaged by the cam 40A.
[0046] FIG. 11 illustrates yet another embodiment of the mechanical
drive with a cam element having a flange at only one side for
engaging the pistons. This embodiment is nearly identical to the
embodiments of FIGS. 8-10 although employing a five-sided cam
element. In this embodiment, like elements are referred to using
similar reference numbers albeit with the suffix "B". Specifically,
a shaft 30B, driven by the motor 9, mounts a separate eccentric 38B
by a dowel pin 100B. A five-sided cam 40B, having five flat outer
surfaces 112, is journalled to the eccentric 388B by a bearing 42B
held in place by a washer 101B and snap ring 102B. Although
five-sided rather than circular, the cam 40B is like that in the
embodiment of FIGS. 8-10 in that it has a flange 32B at one side
that includes five axial tabs 103B that define slots 34B which
receive flanged piston heads disposed in five blocks as shown and
described in the embodiment shown in FIGS. 8-10. Also like the
embodiment of FIGS. 8-10, a generally semi-circular counterweight
110B is mounted to the shaft 30B by the dowel pin 100B or other
suitable means at the short side of the eccentric 388B to balance
the weight of the eccentric 388B and reduce vibration when the
shaft 30B is rotated.
[0047] As in the above embodiments, as the shaft 30B is rotated,
the eccentric 38B orbits around the axis of the shaft 30B. The cam
40B is not allowed to rotate but orbits with the eccentric 38B,
causing the pistons to reciprocate in their corresponding valve
blocks. When the shaft 30B is rotating, the pistons will be
consecutively forced into the pump chambers during their pump
strokes by contact with one of the five flat surfaces 112 as the
eccentric 38B orbits toward each piston. Again like the embodiment
of FIGS. 8-10, springs are not used to retract the pistons since
the flange tabs 103B pull the shoulders of the heads of the pistons
on their suction strokes.
[0048] FIG. 6 graphically depicts the system in hydraulic schematic
circuit diagram form. There are two pumping units 50 and 14 that
are driven by the motor 9. The pumping unit 14 is the main pump,
which includes the three sets of pistons 44 and blocks 46 (or five
sets of pistons 44A and blocks 46A depending on the drive unit
configuration). The pumping unit 50 is a low pressure pump, such as
a gear pump or gerotor pump, for supercharging the pumping unit 14,
i.e., for supercharging the three pumping chambers of the pumping
unit 14. The valve 22 is a four way three position valve which
provides an interface between the tool 12 and the pump 3. When the
pump 3 is not performing work, the valve 22 is set to the center
position, in which position the valve 18 holds the load of the
hydraulic device 12. When shifted to the left, the valve 22 moves
into an advance position in which it directs flow from the pumping
unit 14 to the load 12, and connects the tank 11 to line 40. During
the advance operation, oil is drawn up from the reservoir 11
through the filter 42. The fluid goes through the pumping unit 50
and is supercharged by pumping unit 50 to a low pressure preferably
less than 100 psi, for example about 50 psi, and fed into the
pumping unit 14. Excess flow not fed to the pumping unit 14 flows
through check valve 54 and through orifice 56 and back to tank 11.
The check valve 54 and orifice 56 maintain a relatively constant
pressure between the pumping units 50 and 14, so that unit 14 is
substantially always fed with supercharged fluid. However, the
precharge pressure delivered by pump 50 does vary with motor speed,
because the flow rate delivered by pump 50 exceeds that of pump 14
as the motor speed increases, and the back pressure created by
orifice 56 correspondingly increases up to, for example, 100 psi,
although it could be somewhat higher or lower. This has a
beneficial effect to reduce cavitation at higher motor speeds. One
purpose of check valve 58 is, in case a condition arises in which
pumping unit 50 does not provide a sufficient flow to charge the
unit 14, unit 14 can draw directly from tank 11 through valve 58
and filter 60. A pressure relief valve 62 is used to keep the
pressure of the system to a set maximum level, e.g., 10,000 psi.
With the valve 22 shifted to the advance mode the fluid is pumped
out of the pump 3 and into the hydraulic device 12.
[0049] Shifting valve 22 rightward from the center position places
the pump 3 into retract mode. In this mode, the load 12 is placed
in communication through check valve 66 with the normal fluid inlet
to unit 14 and the normal fluid outlet of unit 50. Also in retract
mode, the direction the motor is driven is reversed, so that the
unit 50, which is a bidirectional pump, pumps toward the tank 11.
The pump 14, which is a uni-directional pump, continues to pump
toward valve 22 even though the drive shaft direction is reversed,
and that flow is directed by valve 22 to tank 11 in the retract
mode. Both units 14 and 50 create a suction which draws fluid
through the check valve 66 from the hydraulic device 12. If the
units 14 and 50 are creating a suction, the check valve 54 will be
closed. If the return pressure exerted by the load is sufficient,
the units 14 and 50 will have to do little, if any, work, since the
pumping power will be provided by the load. If not, however, the
units 14 and 50 will help drain the fluid from the device 12.
[0050] The check valve 58 is also used as a safety device for when
the hydraulic device 12 becomes completely depleted of fluid in the
retract mode. In that event, the valve 66 will close under the
force of its spring and the suction provided by the units 14 and 50
will open the valve 58, thereby circulating the oil from the tank
back to the tank through both units 14 and 50, to avoid running the
units 14 and 50 dry.
[0051] A desirable feature of the variable speed pump 3 is the
ability to limit the flow at the high pressure limit, e.g., 10,000
psi. When the controller detects, by monitoring the current to the
motor, that the pump has reached the pressure limit, e.g. 10,000
psi, the controller is programmed to slow the pump rotation to a
speed just necessary to maintain the pressure at this level. This
greatly reduces the heat generated in the pump 14 and provides
benefits in terms of increased life of the hydraulic fluid and
reduced stress on the components of the pump.
[0052] FIG. 7 shows the flow versus pressure of a typical prior art
two stage pump compared to a pump of the first embodiment of the
present invention with the same pressure limit and flow
characteristics. The first-stage pump of the prior art pump
operates at a high flow until a given pressure, indicated as 1,000
psi, when the first stage bypass valve opens. The second-stage pump
then supplies a much lower flow up to the high pressure limit,
10,000 psi. The new pump uses one pumping unit 14 that will have
variable flow to achieve the maximum flow at any point in the
pressure range. The area between the two curves represents the
added work that the new pump is able to produce over the old
pump.
[0053] Thus, the invention provides an improved hydraulic pump in
which a pumping unit is driven with a variable speed, the speed
being set according to the pressure demanded by the load so as to
yield a relatively constant power output of the pump in terms of
pressure and flow rate. This is accomplished by monitoring the
current (or other electrical characteristic of the motor that
varies with load) of the motor that drives the pumping unit, and
increasing or decreasing the speed of the motor so as to provide a
constant horsepower output of the motor. The motor controller is
programmed to monitor characteristics of the motor current, such as
magnitude and/or phase angle, which are related to the torque load
on the motor, so as to enable the motor speed to be controlled in
accordance with pump pressure without the need for a separate pump
pressure sensor and associated electronics.
[0054] Preferably, a single pumping unit is provided to serve the
load, and to reduce cavitation, the pumping unit is supercharged
with a low pressure source of fluid.
[0055] In addition, the pistons are positively returned by the
drive cam, to eliminate power wasting springs.
[0056] Another desirable feature of the invention is the ability of
the pump to produce suction to return fluid to the pump. This is
accomplished by using a three position, four way valve which in a
retract position communicates the pumping unit to tank and
communicates the load to the input port of the pumping unit. The
motor is also driven in reverse, to reverse the pumping direction
of the supercharging pump. Positive return of the pistons also
contributes to the ability of the pump to produce suction. As such
both pumping units produce a vacuum which draws fluid from the
load, to thereby remove hydraulic fluid from the outlet line
quickly.
[0057] In another preferred feature, the pump detects when the
pressure limit is reached and reduces the flow rate to be just
sufficient to maintain the pressure at the limit. This is
accomplished by programming the motor controller to detect, by
monitoring the current characteristics, when the pressure limit has
been reached, and to reduce the motor speed until the pressure
starts dropping, at which point the motor speed is slightly
increased. This process is continued so that the speed hovers at a
magnitude which is just barely sufficient to maintain the pressure
limit, until the pressure subsides or the pump is turned off.
[0058] A preferred embodiment of the invention has been described
in detail. Many modifications and variations will be apparent to
those skilled in the art. Therefore, the invention should not be
limited to the preferred embodiment described, rather reference
should be made to the following claims.
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